Risk assessment in the Pyrocene – Rapid changes in models and predictions complicate US policies to prioritize forest and fuel management

Increased wildfire activity in the US motivated the USDA Forest Service to update national wildfire simulation data used for risk assessment and prioritizing wildland fuel reduction programs. The update (FSim-2020 to FSim-2023) included revised data on fuels, ignition frequency, fire weather, and recalibration of the FSim model. We overlayed the new simulated fire perimeters with the most recent building footprint data to examine the change in magnitude and spatial patterns of estimated building exposure. We found that the update to FSim-2023 resulted in a 61% increase in simulated building exposure in the East, and a decline of −9% in the West compared to FSim-2020. Overall, Forest Service priorities for management investments at the 100,000-ha scale were consistent between the two simulations in the West, but less so for the East, with newer simulations suggesting that a substantially greater area would require treatments to address the same level of exposure. Fire return level analysis indicated that differences between the prior and newer simulated building exposure varied across predicted recurrence intervals and fire size. Changes in building exposure were related to increases in ignition frequency, burn probability and building counts. A potentially important finding was the overestimation of simulated building exposure compared with observed (+112% West, +790% East) for the same approximate time period. One probable cause for the overestimation was the resampling of the LANDFIRE 30 m to the FSim 270 m, which resulted in a substantial reclassification of non-burnable to burnable pixels, particularly in and around the urban interface. The study underscores the importance of continued federal investment in risk assessment technology and data to keep pace with the rapid growth in wildfire extent and severity under evolving wildfire management policy.